Electrical methods for non-destructive testing and measuring the resistivity of metals are well known. For example, the four point probe method, also known as the potential drop method, and the eddy current method are two widely-practiced techniques for measuring the resistivity of magnetic or non-magnetic metals, respectively.
Eddy current methods and instruments have been widely used for measurements of resistivity on non-magnetic steels and nonferrous alloys, such as aluminum. These measurements have also been used for detecting flaws and cracks in non-magnetic alloys. With this technique, no electrical contact with the sample is required. However, due to the significant influence of geometrical effects, this technique requires careful calibration for articles having complex shapes. In addition, the eddy current technique requires substantial corrections for samples having high relative magnetic permeability and therefore have limited applications for measurements of ferrous alloys or ferromagnetic materials.
The four point probe impedance method is widely used for measurements of resistivity on metal samples. A typical four point probe configuration and circuit schematic are shown in FIG. 1. Both direct current ("DC") and alternating current ("AC") embodiments of this method are known in the art. The four-point probe apparatus typically comprises two pairs of probes arranged in a straight line which make electrical contact with the sample to be measured. Two current probes are positioned at either end of the linear array and two voltage probes are placed between the current probes in a collinear arrangement. A current source, either AC or DC, is connected to the current probes to supply a predetermined flow of current through the sample article and the voltage differential between the voltage probe locations is measured and recorded. The resistivity can be determined from the apparent resistance of the sample region located between the voltage probes is determined by applying the familiar Ohm's Law relation, V=IR, where V is the measured voltage, I is the applied current and R is sample resistance. The resultant measured resistivity value is thus a derived absolute value which is significantly influenced by the specific measurement conditions and specific characteristics of the sample being measured.
The apparent resistivity of the sample which is measured by the four point technique is dependent on sample composition, bulk resistivity of the material, geometry and shape of the part and the presence of physical defects, such as voids, cracks and inclusions within the sample volume which is measured. Thus, the method may be used for quality control of sample lots for checking uniformity of the lot composition by measurement of variations of resistivity between samples. Due to the large changes in resistivity produced by annealing treatments or work hardening of complex alloys, the method has been used to determine the extent of alloy phase transformation, such as the austenitic-martensitic transition in ferrous alloys. In addition, the method has been utilized to test the quality of semiconductor materials, magnetic steels and alloys, transformer steels, electroplated coatings, spot welds to gauge uniformity in sheet steel roll stock.
If a sample part has a standard geometry and the sample composition and microstructure is uniform, the effects of composition, microstructure and shape on a four point probe resistivity measurement can be factored into an apparent baseline measurement values for the specific part and deviations from the baseline value can be used to determine the presence of unacceptable defects within the article. However, for detecting such defects, the four point technique requires careful calibration by measuring the resistivity of an acceptable flaw-free sample which is free of any defects. The voltage value thus measured with a defect-free part is then utilized for comparison with a defective part.
Due to the inherent limitations of typical four point measurement probe configurations, the area and effective volume which is sampled in a four point method measurement is limited by the use of a single or dual two voltage probe pairs of fixed spacing. In establishing an apparent baseline resistivity value, a single four point measurement is typically insufficient for detecting flaws in most articles due to the intrinsic heterogeneity of the sample and limited area sampled by the four point probe. Thus, the four point method requires repeated measurements at numerous points on the sample for calculation of an average baseline value.
An additional limitation in the method is that the measurement of absolute resistivity by this method necessarily assumes a fixed geometrical relationship for the derivation of a resistivity value for a specific probe spacing and sample thickness. Typically, when the actual sample part thickness and probe spacing values do not satisfy such geometric assumptions, the four point method requires application of significant correction factors which lead to further error and lack of precision in determining a sample's resistivity distribution. Such errors will typically mask deviations from baseline measurements due to the presence of flaws in the sample.
Due to the need for measuring and establishing a baseline resistivity value for an unflawed part, the four point probe method has significant additional limitations when used to determine the presence of defects in heterogeneous materials. Due to resistivity variations typically observed in flaw-free materials caused by lot to lot variations in manufacturing methods, annealing treatments, work hardening, grain size, multiple phases, localized compositional inhomogeniety and complex geometry, as routinely encountered in practical applications, such factors tend to mask variations from the baseline resistivity measurement which would enable the detection of flaws in samples. Thus, it is difficult to establish a baseline resistivity measurement with acceptable precision to enable the detection of defects and flaws in such parts. Thus, the calibration required for the four point probe method leads to imprecision and significantly limits the capability of the technique for detecting flaws in conductive articles.
A further limitation of the four point method is that the volume of material sampled in a typical measurement is limited to the material between the narrowly spaced voltage probes. Thus, measurements made by this method are frequently subject to a lack of reproducibility when measurements are made over a large sample surface area on heterogeneous, multi-phase, or porous samples, or where variations in composition or grain size are present. Measurements made by this method on such materials can lead to considerable uncertainty as to what the appropriate baseline resistivity distributors is for an unflawed part. Thus, such variations in measured values preclude the detection of flaws where the perturbation in the measured voltage distribution for a defective part is masked by the scatter and uncertainty in measured values for a flaw-free part.